1. Field of the Invention
The present invention relates to the direct or indirect application of a liquid or viscous coating medium onto a moving surface.
2. Description of the Related Art
In the case of direct application, the coating medium is applied directly to the surface of a moving material web, specifically a paper or cardboard web. In the case of indirect application, the coating medium is first applied to the surface of a moving or rotating transfer element, preferably that of a transfer roll, which then transfers the coating medium to the material web. The coating medium is applied to the moving surface by use of an applicator unit. The volume of coating medium dispensed by the applicator unit within a given time period is adjustable. A metering device includes a metering element upon which an adjustable actuating force is exerted for the purpose of adjusting the metering device against the moving surface. The metering device is located after the applicator unit, when viewed in the direction of travel of the moving surface and whereby the applied coating weight is measured.
A method of this type is described in German patent document DE 196 05 183 A1. In that method, the coating weight measurement signal is subjected to a frequency analysis and is divided into a high frequency signal portion and a low frequency signal portion. The high frequency signal portion is supplied to a regulating device for changing the actuating force that is exerted upon the metering element. The low frequency signal portion is supplied to a regulating device for changing the volume of coating medium which is dispensed in a given time period by the applicator unit.
A coating method is described in German patent document DE 196 37 164 A1 in which the coating weight is determined by influencing the actuating force exerted on the metering element, as well as by influencing the volume of coating medium that is dispensed by the applicator device within a given time period. Either the applicator device or the metering device will take over the longitudinal profiling of the coating layer that is applied to the moving surface while the other devicexe2x80x94metering device or applicator devicexe2x80x94will be responsible for the cross profiling of the coating layer.
The present invention reduces wear and tear of the metering element by comparing the measured coating weight to a predetermined desired value and adjusting both the volume of coating medium that is being applied and the actuating force exerted upon the metering element dependent upon the result of the comparison.
When the measured coating weight exceeds a predetermined desired value, the coating medium volume dispensed by the applicator device within a predetermined time period is reduced. When the measured coating weight drops below a predetermined desired value, the actuating force exerted upon the metering element is reduced.
The contact pressure which the metering element exerts against the moving surfacexe2x80x94on the one hand due to the effect of the actuating force and on the other hand by the coating medium pressing against it (in the instance of a doctor blade the force exerted upon the blade tip)xe2x80x94decreases when reducing the actuating force that is exerted upon the metering element. The contact pressure exerted by the metering device also is reduced when the amount of coating medium dispensed by the applicator device within a given time period is reduced. Wear and tear of the metering element, however, also depends on the magnitude of this contact pressure. If, in reaction to an excessively high coating weight, only the amount of coating medium that is being dispensed by the applicator device within a given time period is reduced, that is while maintaining the same or substantially the same actuating force, the coating weight can be brought back to the predetermined desired value and the contact pressure of the metering element against the moving surface reduces accordingly. If, in reaction to a coating weight which is too low, only the actuating force exerted upon the metering element is reduced, while maintaining the same or substantially the same amount of coating medium being dispensed by the applicator device within a given time period, then the coating weight can be increased to the predetermined desired value, thereby correspondingly reducing the contact pressure exerted by the metering element against the moving surface. Thus, the method of the present invention adjusts to coating weights that are too high or too low in a manner that reduces wear and tear on the metering element.
The value of the actuating force exerted upon the metering element, as well as the amount of coating medium released by the applicator device within a given time period, may only be reduced within certain limits before negatively affecting the smoothness and the uniformity of the applied coating layer. The method of application of the present invention represents a simple method to return the operating parameters during the coating operation to predetermined values (i.e., a xe2x80x9crelaxation modexe2x80x9d) that ensure a good quality coating result while minimizing the wear and tear of the metering element. For example, if a lower limit for either the actuating force exerted upon the metering element and/or for the amount of coating medium dispensed by the applicator device within a given time period is reached or fallen below, then the operating parameters can be adjusted accordingly (i.e. the xe2x80x9crelaxation modexe2x80x9d is exited).
The wear and tear occurring at the metering element can be decreased by reducing the amount of coating medium dispensed by the applicator device within a given time period and increasing the actuating force exerted upon the metering element only when the measured coating weight exceeds a predetermined desired value. Wear and tear occurring at the metering element can also be decreased by increasing the amount of coating medium dispensed by the applicator device within a given time period and decreasing the actuating force which is exerted upon the metering element only when the measured coating weight falls below a predetermined desired value. This is accomplished by keeping the contact pressure of the metering element constant relative to the moving surface by adjusting the actuating force or adjusting the amount of coating medium being dispensed by the applicator device within a given time period. The angle of attack of the metering element against the surface can also be adjusted to reduce wear and tear of the metering element.
It is recognized that a coating weight reduction is achieved by increasing the actuating force exerted upon the metering element, which is associated with an increase in the contact pressure. A reduction in the coating is also achieved by a reduction of the amount of coating medium dispensed by the applicator device within a given time period, which is associated with a reduction in the contact pressure. It is further recognized that a coating weight increase is achieved by either reducing the actuating force exerted upon the metering element, which is associated with a corresponding reduction of the contact pressure, or by increasing the amount of coating medium that is released by the applicator device within a given time period, which is associated with a corresponding increase in the contact pressure. Based on this, when a reduction or an increase in coating weight is desired, a substantially constant contact pressure of the metering element relative to the moving surface is maintained through appropriate control combinations of applicator device and metering element during coating weight increases or coating weight reductions.
This method is especially advantageous in cross profiling of the applied coating layer. In order to achieve the desired cross profile the contact conditions of the metering element against the moving surface are selected such that the contact pressure of the metering element relative to the moving surface is substantially uniform across the entire working width of the metering element, thus resulting in uniform wear across the working width of the metering element. This is of particular advantage because, conventionally, the life span of a metering element is limited by increased wear at only a few locations, resulting in replacement of the metering element when the majority of the working width thereof would still be usable.
Parallel considerations apply to a change in the angle of attack of the metering element against the moving surface. An increase in the adjustment force exerted on the metering element relative to the moving surface results in a stronger deflection of the metering element, or the mounting thereof, and thus the angle of attack of the metering element is reduced. Similarly, a stronger deflection of the metering element, or the mounting thereof and thus a reduction in the angle of attack of the metering element, results from an increase in the amount of coating medium being released by the applicator device onto the moving surface within a given time period. Parallel statements apply relative to a reduction of the adjustment force exerted on the metering element relative to the moving surface and the amount of coating applied.
By appropriate control combinations of applicator device and metering device, a state is achieved where the angle of attack of the metering element against the moving surface remains substantially constant, even during increases and decreases in the coating weight. This has a particularly positive effect on the quality of the applied coating. Most importantly however, the difficult and time-consuming readjustment procedure of the angle of attack of the metering device is avoided.
In practical application, metering element movements resulting from a variation in the force applied by the metering element on the moving surface are normally in a magnitude of millimeters, while the metering element movements resulting from a variation in the coating volume are in a magnitude of micrometers.
The previously discussed operating modes can be combined, i.e., the contact-pressure based operating mode and the angle of attack based operating mode. An example of such a combined operating mode is proportionally determining the actuating signals for the applicator device and the metering device through use of contact pressure oriented control and an angle of attack oriented control.
Yet another operating mode provides, for example, the simultaneous correction of the coating weight and the contact pressure with which the metering element is adjusted against the surface. The amount of coating (or the adjustment power) is adjusted dependent upon any deviation of the contact pressure from a desired value to an increased value. This additional deviation is compensated for by an appropriate change in the amount of coating being applied.
To enhance the measuring accuracy, the coating weight is measured by use of a sensor device. The sensor device may, for example, include at least one first basis weight sensor which would measure the basis weight of the material web prior to coating, at least one second basis weight sensor which would measure the basis weight of the material web after coating, and a subtraction device which calculates the coating weight from the difference between the basis weights measured by the second and the first basis weight sensors.
For cross profiling of the coating application, the sensor device measures the coating weight with local resolution, that is, it always determines a separate basis weight value for a multitude of sections adjacent to each other in a direction transverse to the direction of movement or rotation of the moving surface.
To automate the previously described method, a control unit is provided which, depending upon the measured coating weight, determines controller outputs to change the actuating force exerted upon the metering element and the amount of coating medium being released by the applicator device within a given time period. An input unit is assigned to this control unit with which an operator who has measured a deviation in the coating weight from a desired value inputs whether the coating weight should be increased or decreased. The control unit can alternatively be configured as a controller to which a coating weight signal is supplied from the sensor and which then, based on a database of desired values or desired value profiles, independently makes any required adjustments to the applicator device and the metering device, and monitors the success of the implemented adjustments on the basis of newly supplied coating weight signals.
The metering element is, in the embodiment shown, a doctor blade. However, the metering element can be alternatively configured as a smooth or profiled metering rod. All methods of the current invention may also be utilized to influence the cross profile, as well as the longitudinal profile of the coating layer.